1. Field of the Invention
The present invention relates to a water level measuring apparatus for measuring a water level of a dam or a river.
2. Description of the Related Art
The water level of a dam (or a river) is an important datum for watching an overflow from the dam and a water quantity left for water-utilization when the water level is high and low respectively. Therefore, in order to measure the water level, the water level measuring apparatus is required to have very high accuracy such as plus or minus one centimeter, over a large depth of the dam like several tens of meters.
As a related art, there have been two types of water level measuring apparatus. One is a pressure type water level gauge which measures the water level by detecting the water pressure proportional to the water level at a bottom of the dam, and the other is a float type water level gauge which measures the water level by detecting a position of a float moving up and down with a water surface of the dam. Recently, the pressure type water level gauge is used widely, because, the pressure type water level gauge can be provided in cheap and less installation costs, compared with the float type water level gauge.
However, the pressure type water level gauge has a defect that the more the water level becomes high, the more an error increases. Therefore, when the dam is deep as much as several tens of meters, the float type water level gauge has been used though a lot of costs are required, in particular, for the installation thereof.
The float type water level gauge has accuracy as high as plus or minus one centimeter. FIG. 1 shows a schematic illustration for explaining a float type water level gauge 6 installed in the dam. The float type water level gauge 6 consists of a water level detection unit 61, a signal converter 62 and an electrical wire for connecting water level detection unit 61 with signal converter 62. The water level detection unit 61 consists of a pipe unit 611, a float unit 612 moving up and down in pipe unit 611 with the movement of a water surface 100 of the dam and a level signal generator 613 for generating an analog water level signal corresponding the position of moving float unit 612. The analog water level signal is sent from level signal generator 613 to signal converter 62 and converted to a digital water level signal which is an output signal of float type water level gauge 6.
The float type water level gauge 6 has a merit of having high accuracy in the measurement of the water level. This is because of a feature of the float. However, float type water level gauge 6 has a demerit of requiring a great deal of costs for installation. As shown in FIG. 1, a large size of water level detection unit 61 must be fixed to a bottom 201 after the severe investigation of the farm ground for the installation. When the dam has a depth of 50 m for example, water level detection unit 61 having the height of more than 50 m and the inner diameter of 600 mm must be installed in the dam, which results in requiring a great deal of manufacturing and installation costs.
Meanwhile, the pressure type water level gauge is widely used recently, because, it can be installed at any place with less installation costs, compared with the float type water level gauge. FIG. 2 shows a schematic illustration of a pressure type water level gauge 1 installed in the dam. In FIG. 2, the same reference numeral as in FIG. 1 designates the same thing as in FIG. 1. The pressure type water level gauge 1 consists of a water pressure detector (DET) 11, a signal converter (CONV) 12 and an electrically connecting wire for connecting DET 11 with CONV 12. The DET 11 is placed on bottom 201 and an element such as quartz is provided therein so as to produce an analog signal proportional to the water pressure or the water level of the water in the dam. The analog signal from DET 11 is sent to CONV 12 through the connecting wire and converted into a digital signal represented by, for example, four figures in the BCD (Binary Coded Decimal) system. The pressure type water level gauge 1 is convenient in use with less installation costs, however, produces the larger error, compared with float type water level gauge 6. The pressure type water level gauge 1 produces at least 0.1% of a relative error. Therefore, when DET 11 is placed on bottom 201 in the dam having 50 m depth, 5 cm of the relative error is produced. This 5 cm of the error is great in case where the dam has a large scale. Considering such large error, the pressure type water level gauge is inadequate for measuring the water level of the deep dam. Therefore, in order to measure the water level in such deep dam, two ways, a first way and a second way, have been taken in the prior art. The first way is of using the float type water level gauge over the whole depth. The second way is of separating the depth into high and low regions so that the float type water level gauge and the pressure type water level gauge are used for the high and the low region respectively. The first way is excellent because of the high accuracy but requires too much costs for manufacturing and installation as explained before. The second way is reasonable in accuracy and costs but unreasonable in operation reliability because of too much complexity in constitution.